1. Field of the Invention
The present invention relates to encoders, and more particularly to an encoder that optically detects positional information of a movable body.
2. Description of the Background Art
As an encoder by a scan method, an encoder has been proposed that irradiates an illumination light, which is modulated based on a predetermined modulation signal, on a scale that moves with a movable body and has a grating periodically formed along a movement direction, and compares a reflected light or a transmitted light of the illumination light with the modulation signal, thereby detecting positional information of the scale (e.g. refer to U.S. Pat. No. 6,639,686).
In the encoder by a scan method, since the incidence position of the illumination light with respect to the scale needs to be periodically changed, for example, an oscillating mirror (a scanner) or the like used to oscillate the illumination light in a direction orthogonal to its propagation direction needs to be placed in an optical path of the illumination light. Therefore, in this type of the encoder, a space used to install the oscillating mirror or the like becomes necessary, which causes the inconvenience such as the increase in size and cost of an apparatus.